Semiconductor devices commonly include an electrically conductive gate electrode that is insulated from an adjacent semiconductor body by a gate oxide. During operation of the device, an electrical potential applied to the gate electrode develops across the gate oxide and controls electrical current flowing in a channel region of the semiconductor body.
Certain applications, such as power switching applications, require substantial switching voltages, which may cause device failure if not properly accounted for. Over time, dielectric materials used to form gate oxides tend to break down when subjected to excessive electric fields. Thus, the regions of the gate oxide in which the electric fields are highest represent the regions that are most vulnerable to dielectric breakdown.
The problem of dielectric breakdown may be compounded by physical defects in the gate oxide. Some of these defects occur from mechanical stresses exerted upon the gate oxide. For example, the high temperatures associated with device fabrication and operation may result in undesirable thermal expansion of the material adjacent the gate oxide. This thermal expansion may sufficiently deform the gate oxide such that performance is reduced (e.g., by increased leakage currents) or may cause complete device failure.
One process technique that addresses the above described problems involves setting a minimum gate oxide thickness that is closely monitored during device manufacturing. Gate oxide thickness plays an important role in device performance. Thickening the gate oxide decreases the capacitance but increases the on-state resistance of a device. The threshold voltage (Vt) of a device is a function of gate oxide thickness and the doping level of the semiconductor material in the channel region. Thus, the only way to thicken a gate-oxide and achieve a targeted threshold voltage is to reduce the doping level in the channel region, which in turn increases the on-state resistance of the device. Thus, adjustments to gate oxide thicknesses of devices result in tradeoffs between performance and reliability.